Category Archives: observation

Refraction of light in water.

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What water can do to light.

In the last post, I showed you a couple of pictures of a vase filled with dandelions.

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Dandelions and light being focussed by the water in the vase.

Turns out this might not have been enough of a clue, so here we go:

Isn’t it amazing time and time again how water refracts light and makes things look distorted?

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Refraction of light in water.

This can be used for all kinds of cool experiments, provided you have the right kind of coins at hand:

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1 NOK with a water droplet in the middle, acting as a lens. See how the dandelions are flipped upside down in the middle of the coin?

This kind of stuff keeps me entertained for quite some time!

Standing waves

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Standing waves caused by rocks in a current.

I am incredibly fascinated by standing waves.

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Standing waves.

The standing waves are caused by rocks sitting in a current. From the pictures below it is not really clear where those rocks are situated, whether they are upstream of all this wave action or in the focal point of the wave fronts.

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More standing waves.

Having stood there with my mom for quite some time the other weekend, just watching the water, I can tell you that it’s the upstream obstacle. You can see for yourself here:

What you also see in that video is that not all of the waves are, in fact, standing waves. The lower-amplitude waves to the left on both the image above and below are not – they are radiating away from some obstacle.

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More standing waves.

Just from looking at that image it is clear that the bathymetry is very irregular and that the current speed is quite inhomogeneous, too. So maybe it is not surprising that the condition for a standing wave – that the current speed and the wave speed are the same, but going in opposite directions – is not met everywhere. Particularly, in many cases it is hypercritical and the waves are just flushed away. Note the current speed in the video below.

And all of this action is happening on an exciting river called … wait for it … Pinnau. In Mölln. And this is what it looks like to most people: Tiny little rapids somewhere in a forest.

 

Pinnau

P.S.: I just realized that when I’ve talked about standing waves before on this blog, I’ve always talked about the see-sawing kind. When obviously this kind is so much cooler!

Wake of a house.

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Am I weird for noticing that kind of stuff?

When I posted that picture of the fountain in the last post, it very strongly reminded me of a breakfast my sister and I had in Shetland in 2009, where the flags on the two poles outside the window were blowing towards each other (clearly caught in the wake of the house). I remember me mentioning it to her and taking pictures of it then, but even though I have looked through hundreds of pictures from that epic holiday, I can’t find the pictures that I remember taking through the window. But what I found instead is that I took pictures of the phenomenon from the outside. On three different days!

Day 1.

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Shetland flags outside a youth hostel in Lerwick, Shetland.

Day 2.

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Same house, same flags, different day.

Day 3.

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Same house, same flags, third day.

Do you see what I mean? How weird and fascinating is that??? (And how weird and slightly disturbing is it that I vividly remember those flags that we saw 5 years ago?)

But seriously. Doesn’t that make you wonder what the story behind those flag poles might be? Did they put up one and then noticed after a while that it never showed the wind direction that the flags down at the harbor showed, so they put up the second one on the other side of the house? Did they realize right away that flags on the lee side of a house were going to do something weird, so in order to show that they put two poles? Is there a hidden camera somewhere, waiting to capture people’s reaction to the flags? So many questions…

Wind shear.

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Wind going in different directions at different heights.

Apparently some people were intrigued by the white mist they saw on the last picture in my last post (below).

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Lombardsbrücke.

First, let me tell you one thing: You clearly need to come visit Hamburg.

But now let’s solve the mystery. This is what is causing that mist:

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Fountain on Binnenalster. Surely by now you recognize Hamburg town hall in the background?

And while we are talking about that fountain, have a look at that picture below.

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Wind shear.

The top of the fountain blowing to the left while the bottom is blowing to the right. How awesome is that???

—

One last picture for those of you wondering about the “behind the blog” of this blog. How do I find the time to take all the pictures? The answer is – I never go somewhere with the specific purpose of taking pictures for my blog. The pictures from the last two posts were taken on a Saturday trip with my parents and my granddad. We were happily sailing along, enjoying the views and the sunshine, and I took pictures of whatever caught my eye.

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Me in front of Hamburg town hall. Picture taken by my mom on the same trip as all the pictures in this and the last post.

And I think that’s the main reason why the massive project of writing a blog is surviving even though I am pretty busy with, you know, work and life: Because I just love seeing how maybe not every single day, but at least every week I am seeing something amazing (to me!) related to ocean sciences. It’s like other people might have their #100happydays projects or that kind of thing – a constant reminder of how much I enjoy ocean sciences and, more importantly, how much ocean science there is in my life still, even though I am not at sea nearly as much any more as I would like to be.

Reading the water

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Just because it’s fun! :-)

I’ve mentioned before that I tend to stare at water when nobody else seems to find anything interesting to look at. So just because I’m weird, let’s look at some more water.

For example here. What could have caused waves like those below?

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What could have caused this pattern?

Yes. These guys went past and what we see are both the circular waves caused by the oars and the stern wave of the boat.

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Rowing boat. Seriously, why would anyone want to go backward all the time???

Ok. So on to the next riddle: What could cause what we see below?

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Bubbles on water. What could have caused them?

Right, that was him:

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Alsterdampfer!

And this?

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More waves.

Yes! Him again!

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Alsterdampfer.

Does anyone see where we are going with this?

Correct. Here.

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Hamburg town hall.

And a last glimpse on the way back:

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Lombardsbrücke.

Isn’t this the most beautiful city in the whole wide world? :-)

Lava

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Don’t you just love lava lamps?

I got a lot of weird looks when I excitedly told people about two years ago that I had just bought a lava lamp. But what’s not to love about them? Plus they are great for teaching. These days kids don’t know them any more, so they are missing out on a really nice mental image of how convection works. Be it in the Earth’s mantle or in the ocean…

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“Lava” flow.

When I moved into my new flat, for the first week I only had an inflatable air bed and my lava lamp in my living room (oh, and a lot of boxes of course). So I have spent a lot of time looking at how the flow changes over time.

Today, all I want to share is this 9 minute movie of the lava lamp. But I’m working on a post where I’m discussing the temporal development of the flow. Sounds interesting? Stay tuned! :-)

Wind waves meet current

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Wind waves on one side of the current – no waves on the other.

Recently in Bergen, I was walking to meet up with a friend at the kayak club, and I had to cross a bridge that has always fascinated me. Underneath the bridge, there is only a very narrow opening connecting basically the ocean on one side and a small bay on the other side. On this part of the Norwegian coast, the tidal range is easily of the order of a meter, so this narrow opening under the bridge makes for some pretty strong currents. In fact, when paddling through that opening, when the tide is right you can really see how the surface elevation changes from one side of the bridge to the other.

So when I was walking there recently, this is what I saw:

Strong current from the lower left to the upper right of the picture, wind blowing from the right, hence waves on the right side of the current and no waves on the left side.

This might be difficult to see on this picture, but there is a strong current going from the lower left corner of the picture towards the upper right. And on the right side of that current there are a lot of wind waves. But on the left side there are hardly any, even though there is nothing blocking the wind, just the current blocking the propagation of waves. Wind is coming from the right here.

I found it really fascinating how this current acted as a barrier to the waves and stood a couple of minutes watching. A couple of people stopped and looked, too, but didn’t find anything interesting to see and were slightly puzzled. But what I see is fetch (or that there isn’t enough of it on the left side of the current) and hydraulic jumps (or that the current is clearly going faster than the waves are). Which means that I start wondering how fast that current would have to be in order to stop waves from propagating across. Which then means I start estimating the wave lengths in oder to estimate the waves’ velocities to answer the previous question. So that’s reason enough to stand there for quite some time, just watching, right?

Advection fog

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When warm, moist air is advected and brought in contact with colder surfaces.

Recently I’ve been starting to think about a course I’ll be teaching later this year, and how it would be cool to have household examples for most, if not all, of the topics I’ll be talking about.

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Fogged up bathroom window

So this is one example for advection fog: Warm, moist air moves against a cold window and condenses.

Of course you can also observe this over other cold surfaces, for example over the ocean:

In the movie below you can witness how the iceberg slowly vanishes as the fog closes in on the ship.

It can actually get pretty spooky.

On this picture you can clearly see that the fog is confined to a shallow layer directly above the ocean’s surface. We were standing on the deck above the bridge, and there we were up high enough to see that it is indeed a thin layer and that the skies above are blue. From the working deck it felt like fog had swallowed us up and the Black Pearl was about to appear…

Hydraulic jump II

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More movies of my kitchen sink.

I am really fascinated by the hydraulic jumps in my kitchen sink. I can’t believe I haven’t used this before when I was teaching! Yes, movies of rivers and rapids are always really impressive, too, but how cool is it to be able to observe hydraulic jumps in your own sink? Let me remind you:

Hydraulic jump in my kitchen sink. Video here

So this is what happens when the water jet hits the (more or less) level bottom of the sink. But what would happen if it instead hit a slope?

Now, if I wasn’t working a full-time job, or if that job wasn’t completely unrelated to anything to do with hydraulic jumps, I would now proudly present movies of all kinds of hydraulic jumps on sloped surfaces. As it is, I can tell you that I have tons of ideas of where to go to make really nice movies, but for now this is all I can offer:

Yes, that is a chopping board in a sink. It shows really nicely how the hydraulic jump occurs closer to the point of impact of the jet as you go uphill (because the water slows down faster going in that direction than going downhill) and again how the radius depends on the flow speed of the jet. Stay tuned for a more elaborate post on this!

Hydraulic jumps

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Water changing its velocity from above to below the critical velocity.

Recently in beautiful Wetzlar: The river Lahn flows through the city below the medieval cathedral at sunset. And I’m showing you this because we can observe a hydraulic jump!

A hydraulic jump occurs when water that was flowing faster than the critical speed suddenly slows down to below the critical speed. Some of its kinetic energy is converted to potential energy (see the higher surface levels of the turbulent part of the fluid {except in this example the water is flowing down a steep slope, so the higher levels are a bit tricky to observe}) and a lot of energy is lost to turbulence. A very nice example can be seen here:

As the water moves away from where the jet hits the sink, it slows down. Can you spot the hydraulic jump? Isn’t it cool to watch how it is pushed away if the flow rate is higher, and how it comes back again when the tap is slowly closed?

P.S.: Yes, I’m being very vague about what that critical speed might be. Stay tuned for a post on that, I’m working on it! Just had to share the Lahn movie :-)